1. Field of the Invention
The invention relates to a projector, and more particularly, to a projector capable of projecting a gray-level image for adjusting a color wheel delay.
2. Description of the Prior Art
Generally speaking, there are two kinds of projectors, specifically, a projector with one image modulator and a projector with three image modulators. The projector with one image modulator focuses light from a light source on a color wheel with red, green, and blue colors through a lens, separates the light into red light, green light, and blue light by rotating the color wheel at high speed, harmonizes the color light with a relay lens, reflects the color light on the image modulator with a prism, controls the image modulator to operate synchronously with the color wheel with a control circuit, and projects the colored light on a screen. The projector with three image modulators divides a light source into red light, green light, and blue light; modulates the red light, green light, and blue light separately with three image modulators; combines the red light, green light, and blue light to form a colored image; and projects the colored image on a screen through a projection lens. Because the projector with one image modulator reflects the red light, green light, and blue light on one image modulator, colors and brightness of the image projected by the projector with one image modulator are not as good as those of the image projected by the projector with three image modulators. However, the projector with one image modulator has the advantages of smaller size, low weight, and portability. With the technical advance of the image modulator, most projectors are designed with one image modulator.
The digital light processing (DLP) projector uses a technique developed by the Texas Instrument Corporation. The DLP projector comprises a digital micromirror device (DMD) that forms and projects an image by reflecting light with reflection mirrors on the digital micromirror device chips. The digital micromirror device has the qualities of sensitive reaction and full utilization of light, which are necessary for projectors with a single image modulator, therefore, projectors with one image modulator are usually implemented with the DLP projector.
FIG. 1 is a diagram of a projector 10 with one image modulator according to the prior art. The projector 10 comprises a light source 12, a lens 14, a color wheel 16, a relay lens 20, a sensor 18, a mark 17, a motor 15, a prism 22, an image modulator 24, a control circuit 26, and a camera lens 28. The color wheel 16 comprises a red filter, a green filter, and a blue filter. The motor 15 rotates the color wheel 16 and the sensor 18 senses the mark 17 on the color wheel 16 to detect the position of the colored filters. The light source 12 emits light and the light is focused via the lens 14. The light passes through the color wheel 16 and the relay lens 20 and is reflected on the image modulator 24 via the prism 22. The control circuit 26 controls the image modulator 24, transmits a red image, green image, or blue image according to the light passing through the red filter, the green filter, or the blue filter, and projects the image on a screen 30 via the camera lens 28. When the image modulator 24 transmits a colored image, it should operate synchronously with the color wheel 16 so as to precisely project the color passing through the colored filter areas. The sensor 18 detects positions of the machine axis or the motor armature. However, no matter whether an electronic or mechanical method is used, an inaccuracy still occurs and, therefore, adjusting the image modulator 24 to operate synchronously with the color wheel 16 is necessary. The value adjusted is the color wheel delay.
The projector 10 of the prior art adjusts the color wheel delay by displaying a predetermined testing curtain. For example, a red curtain, green curtain, or blue curtain could be used. The projected image is captured via an additional optical sensor and transformed into analog signals. The wave pattern of the analogy signals is displayed with an oscilloscope. The color wheel delay is adjusted according to the difference between the wave pattern displayed by the oscilloscope and the ideal wave pattern. The adjustment allows the image modulator 24 to operate synchronously with the color wheel 16 and the red light, green light, and blue light projected by the projector 10 is not mixed with another color light.
As described previously, the projector 10 with one image modulator 24 focus the light on the rotating color wheel 16 to separate the light into red light, green light, and blue light. The image modulator 24 modulates the red image, green image, or blue image using the light passing through the rotating color wheel 16. By adjusting the color wheel delay of the projector 10, the image modulator 24 operates synchronously with the color wheel 16 so that the projector 10 precisely projects the color passing through the color wheel 16. According to the prior art, the additional optical sensor and the oscilloscope are necessary to adjust the color wheel delay, however, these devices are expensive, not commonly available, and inconvenient.